The crystal structure of the 160-nucleotide P4-P6 domain of the Tetrahymena self-splicing intron, recently solved in my laboratory, is the largest, and one of the few, crystal structures solved to date for RNA. Thus, many fascinating and unexpected chemical interactions have been identified that explain biochemical observations and functional properties of the intron RNA. The domain is characterized by a ~180x bend that allows side-by-side packing of adjacent helical regions. Two sets of tertiary contacts stabilize this fold, one between an adenosine-rich bulge and the minor groove of a distal helix, and the other between a GAAA tetraloop and its receptor. Many metal binding sites are seen in the structure, explaining the importance of magnesium in domain folding. In addition, 2' hydroxyl-mediated contacts abound, epitomized by ribose zippers that occur between closely-packed helical backbones. The structure of this RNA was solved by multiwavelength anomalous diffraction (MAD) phasing using an osmium hexammine derivative. Four osmium hexammine binding sites occur in each of the two non-identical molecules in the asymmetric unit of the crystal. High resolution data sets collected at CHESS were essential to produce the electron density maps used for model building and ongoing refinement of the structure.